The general forms of most metal detectors which interrogate soils are either hand-held battery operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held battery operated units include detectors used to locate gold, explosive land mines or ordnance, coins and treasure. An example of a conveyor-mounted unit is a fine gold detector used in ore mining operations, and an example of a vehicle-mounted unit includes a detector to locate buried land mines.
These electronic metal detectors usually include transmit electronics for generating a repeating transmit signal cycle, which is applied to an inductor, for example a transmit coil, which transmits a resulting alternating magnetic field (also known as a transmit magnetic field).
Time-domain metal detectors usually include switching electronics, within the transmit electronics, that switches various voltages from various power sources to the transmit coil for various periods in a repeating transmit signal cycle.
Metal detectors contain receive electronics which processes a receive magnetic field to produce an indicator output signal, the indicator output signal at least indicating the presence of at least some metallic targets under the influence of the transmit magnetic field.
Traditional PI (Pulse Induction) metal detectors are time-domain detectors, having a plurality of switches for switching at least first and second voltages from power sources, and zero volts for various durations, to generate a repeating transmit signal cycle with a fundamental frequency usually being in the range from tens of Hz to several kHz. The second voltage from a second power source is usually a low negative voltage, −6V for example, and is switched to the transmit coil during a low-voltage period, immediately followed by a back-emf period (a high-voltage period) of a first high voltage, for example +180V, switched from a first power source to the transmit coil usually via a forward-biased diode, then a zero-voltage period following the high-voltage period. The transmit electronics presents a low source impedance to the transmit coil during the low-voltage period and back-emf period, assuming that the coil is connected to the first power source, but presents a high impedance during the period in which critically damped decay of the back-emf occurs, and during the zero-voltage period when no transmit coil current flows and a magnetic signal is received by the receive electronics. During these high impedance periods, the said switching electronics output impedance is usually a function of the capacitance of the switching electronics in parallel with a resistor (e.g. 500Ω) whose value is usually selected to critically damp the self-resonance of the transmit coil connected to transmit electronics. As this period of relatively high impedance commences with a decay of a pulse induction back-emf period, the receive signal will contain a reactive component (X) during this decay period. Hence, to avoid contaminating the receive signal with this X component, usually most, if not all, of the receive signal processing of sampling, or synchronous demodulation, is delayed so as to occur during the period of zero-voltage after the back-emf has substantially decayed.
Eddy currents induced in metallic targets, such as small gold nuggets and fine gold chains, have short decay periods. The delay of the sampling, or synchronous demodulation, of the receive signal after the back-emf periods results in reduced sensitivity to those targets harbouring eddy currents with short time constants. However, the delay cannot be made too short because contamination of the receive signal with X component occurs if the receive sampling occurs when the transient output from the receive coil is significant. Hence, if the duration of the transient output from the receive coil can be reduced, the delay can be reduced, and targets with faster time constants targets can be detected without contamination from X.
A possible solution is detailed in WO2009/155668 where the repeating transmit signal cycle is monitored and controlled such that the receive signal is processed during a zero reactive voltage period during which a constant non-zero current is flowing through the transmit coil. As it is a zero reactive voltage period, there will be no contamination from X when the receive signal is processed. To create such a zero reactive voltage period, WO2009/155668 discloses a switched rectangular repeating transmit signal cycle.
The present invention teaches an alternative to the switched rectangular repeating transmit signal cycle.